With respect to methods of producing color filters to be used in a liquid crystal display element and a solid pickup element, there are known a dyeing method, a printing method, an electrodeposition method and a pigment dispersing method.
In the case of adopting a dyeing method, a color filter is produced by dyeing a base material comprised of a natural resin, such as gelatin, glue or casein, or a synthetic resin, e.g., an amine-modified polyvinyl alcohol, with an acid dye or the like.
One problem of such a dyeing method is that the color filters produced are not good in light resistance, heat resistance and moisture resistance because dyes are used therein, and other problems thereof are, e.g., that a large-sized screen is apt to be dyed unevenly because of a difficulty in controlling so as to render dyeing and adherence characteristics uniform over the screen all, and that the printing resist layer is required at the time of dyeing to complicate the production process.
In the case of using the electrodeposition method, the process of producing a color filter comprises steps of forming in advance a transparent electrode having an intended pattern, ionizing a pigment-containing resin dissolved or dispersed in a solvent and applying a voltage thereto to form the intended pattern of colored image.
Such an electrodeposition method also has some problems. For instance, it requires a photolithographic process, which comprises the formation of a transparent electrode film for producing a color filter and the etching step, in addition to the step of forming a transparent electrode for display. The short formed in the foregoing process causes a linear defect to result in the lowering of yield rate. Further, it is difficult in principle to apply such a method to a mosaic arrangement and others, excepting a stripe arrangement, and the maintenance of transparent electrodes is not easy.
The printing method is a simple method for producing a color filter, wherein the printing operation according to an offset printing technique or the like is performed using an ink containing a pigment dispersed in a thermosetting or ultraviolet-curable resin. However, the ink usable therein is difficult to filter because of its high viscosity, so that the color filter produced tends to have defects arising from dust, foreign matter and gelled part of the ink binder. Another problem of the color filter produced is that the precision in position and line-width, which depend on the precision of the printing technique used, and the surface smoothness are not good.
In the case of using a pigment dispersion method, a color filter is produced from radiation sensitive colored compositions, or various photosensitive compositions in which pigments are dispersed, in accordance with photolithography. The color filter produced therein is stable to light and heat since pigments are used, and further the patterning is sufficiently precise in position because it is carried out by photolithography. Therefore, this method is well suited for the production of a color filter applicable to wide-screen high definition color display.
Although the dispersion of pigment in a light-sensitive polyimide resin is disclosed in JP-A-60-237403 (The term "JP-A" as used herein means an "unexamined published Japanese patent application"), the light absorption develops in the visible region when the polyimide resin film is at least 1.0 .mu.m in thickness, thereby causing a color reproducibility problem.
When the pigment dispersibility in a color filter produced using a pigment dispersion method is unsatisfactory, not only deterioration in color purity and dimensional precision becomes a problem, but also the display contrast is considerably lowered due to a depolarizing action. Further, the pigment-dispersed radiation sensitive composition produces aggregates with the lapse of time, thereby lowering the coatability and causing the aforementioned problems in the color filter produced therefrom.
As a means of heightening the pigment dispersibility, JP-A-1-102429 discloses the method of using a processed pigment obtained by treating a pigment with an acrylic resin, a maleic acid resin, a vinyl chloride-vinyl acetate resin or the like, and JP-A-2-181704 and JP-A-2-199403 disclose the method of using organic dye derivatives as a dispersing agent. However, those methods cannot bring about satisfactory solutions for the aforementioned problems.
Also, a surfactant-utilized dispersion method is known. However, this method has a defect that a reaction occurs between the surfactant and the pigment or resin used therein or the surfactant separates out in the course of using the color filter.
Further, the method of dispersing a pigment by using the combination of an ionic surfactant with an ionic resin having the same polarity as the surfactant is disclosed in JP-A-4-76062. As it is, such a method fails in producing satisfactory results.
On the other hand, JP-B-4-39041 (The term "JP-B" as used herein means an "examined Japanese patent publication") discloses the method of producing a color filter by the use of a pigment having a particular grain size. In order to obtain pigment grains having a particular size, centrifugation of pigment grains and filtration of the centrifuged grains through a glass filter or membrane filter are performed therein. According to such a method, the pigment grains of an intended size are obtained, but the production process comprises complicated steps and has an inferior efficiency.
JP-A-7-140654 proposes the radiation sensitive composition comprising a resin prepared from an alkali-soluble monomer, a monomer having an alcoholic hydroxyl group and a macromonomer of styrene and methylmethacrylate. Such a resin is however unsatisfactory in pigment dispersibility.
The production of a color filter according to a pigment dispersion method can be effected as follows: A radiation sensitive composition is coated on a glass substrate by means of a spin coater or roll coater, and dried to form a film. This film is exposed to light through a pattern, and then developed to obtain a colored image. These operations are performed for each color. In the formation of pixels by the use of conventional radiation sensitive compositions, however, the pixels are liable to come off the substrate during the development or washing operation; as a result, the color filter produced is likely to have defects. If the adhesiveness of the pixels to the substrate is heightened as a measure to prevent the aforementioned phenomenon, however, the color filter obtained is likely to have a scum problem and the like because the solubility of non-image areas upon development is lowered.